Electric Potential
Physics 2415 Lecture 6
Michael Fowler, UVa
Today’s Topics
• Some reminders about gravity: mgh and its
electric cousin
• Inverse square law and its potential
• Field lines and equipotentials
Lifting a Rock
• Near the Earth’s surface,
the gravitational field
vector points vertically
down, and has constant
magnitude g, the force on
a mass m is F  mg.
• The work done in lifting
mass m through height h is
mgh: this is the potential
energy.
• a
g
F  mg
h
ground
Lifting a Rock
• The work done in lifting
mass m through height h is
mgh: this is the potential
energy—defined to be zero
at ground level, but could
take some other level as
zero, only differences of
potential energy matter.
• The PE per unit mass, gh, is
called the (gravitational)
potential.
• a
g
F  mg
h
ground
Lifting a Rock along a Wavy Path
• Suppose we lift up the heavy
rock erratically, following the
wavy green path shown. Our
work against gravity only
involves the component of
the gravitational force
pointing along the path:
• a
g
b
F  mg
b
W   mg  d
h
a
• Or, equally, only the upward
component of d counts,
and W = mgh.
a
ground
Electric Potential of a Negative Sheet
• Imagine an infinite sheet of
negative charge,  C/m2.
• On either side of the sheet there
is a uniform electric field,
strength E   / 2 0 , directed
towards the sheet.
• To move a + charge q from the
sheet distance z takes work qEz.
• The electric potential difference
V  z  V  0  Ez   z / 2 0
and this x q becomes KE if the
charge is “dropped” to the sheet.
• a
E
charge q
F  qE
z
negatively charged infinite sheet
Potential, Potential Difference and Work
• We’ve seen that the electric field of a uniform
infinite sheet of negative charge is constant, like the
Earth’s gravitational field near its surface.
• Just as a gravitational potential difference can be
defined as work needed per unit mass to move
from one place to another, electric potential
difference is work needed per unit charge to go
from a to b, say.
• The standard unit is: 1 volt = 1 joule/coulomb
Potential Energy of a Charge Near an
Infinite Plane of Negative Charge
a
Plane of negative Charge
(perpendicular to screen)
PE qV(z) for a positive charge
0
z
PE qV(z) for a negative charge
Electric Field and Potential between
Two Plates Having Opposite Charge
• Separation d is small compared with the • a
size of the plates, which carry uniform
charge densities  .
• The electric force on a unit charge
between the plates E   /  0 N/Coul.

• The voltage (potential difference)
between the plates is the work needed to
take unit charge from one to the other,
V  Ed
• Note from this that E can be expressed in
volts/meter.
d

E   / 0
Units for Electric Potential and Field
• Potential is measured in volts, to raise the
potential of a one coulomb charge by one
volt takes one joule of work:
• One volt = one joule per coulomb
• An electric field exerts a force on a charge,
measured in newtons per coulomb.
• Since one joule = one newton x one meter,
electric field is equivalently measured in
volts per meter.
Review from Phys 1425 Lec 14
Gravitational Potential Energy…
0
• A
• …on a bigger scale!
• For a mass m lifted to a
point r from the Earth’s
center, far above the Earth’s
surface, the work done to
lift it is
r
 1 1
GMm
W   2 dr  GMm   .
r
 rE r 
rE
• If r = rE + h, with h small,
r  rE GMmh
W  GMm

 mgh.
2
rrE
rE
rE
r
U(r) = -GMm/r
In astronomy, the
custom is to take the
zero of gravitational
potential energy at
infinity instead of at
the Earth’s surface.
Electric Potential Outside a Uniformly
Charged Spherical Shell
• Recall the electric field is
1 Qrˆ
E r  
4 0 r 2
precisely the same form as
in gravitation—except this
points outwards!
• Therefore the PE must also
have the same form—
taking it zero at infinity,
1 Q
V r  
4 0 r
• a
V(r)
0
r0
r
Electric Potential Inside a Uniformly
Charged Spherical Shell
• The electric field inside a
spherical shell is zero
everywhere—so it takes
zero work to move a
charge around. The gravity
analog is a flat surface: the
potential is constant—but
not zero, equal to its value
at the surface:
1 Q
V r  
for r  r0
4 0 r0
• a
V(r)
0
r0
r
Potential Outside any Spherically
Symmetric Charge Distribution
• We’ve shown that for a uniform spherical shell
of charge the field outside is
1 Q
V r  
4 0 r
• Any spherically symmetric charge distribution
can be built of shells, so this formula is true
outside any such distribution, with Q now the
total charge.
• It’s true even for a point charge, which can be
regarded as a tiny sphere.
Potential Energy Hill to Ionize Hydrogen
• The proton has charge +1.6x10-19C, giving rise to a
potential
19
1 Q
1.6

10
9
V r  
 9 10 
4 0 r
r
• Taking the Bohr model for the ground state of the H
atom, the electron circles at a radius of 0.53x10-10m, at
which V(r) = 27.2 V.
• The natural energy unit here is the electron volt : the
work needed to take one electron from rest up a one
volt hill. But in H the electron already has KE = 13.6eV,
so only another 13.6eV is needed for escape.
Potential Energy Hill for Nuclear Fusion
• If two deuterium nuclei are brought close
enough, the attractive nuclear force snaps
them together with a big release of energy.
• This could solve the energy problem—but it’s
hard to get them close enough, meaning
about 10-15m apart.
• Each nucleus carries positive charge e, so
19
1 Q
1.6

10
6
V r  
 9 109 

10
eV
15
4 0 r
2 10
• This is the problem with fusion energy…
Potential Energies Just Add
• Suppose you want to bring one
charge Q close to two other fixed
charges: Q1 and Q2.
• The electric field Q feels is the
sum of the two fields from Q1, Q2,
the work done in moving d is
• a
y
Q
V  r   V1  r   V2  r 
Q1
r
r2
E  d  E1  d  E2  d
so since the potential energy
change along a path is work done,
r1
Q2
0
x
1  Q1 Q2 
V r  
  
4 0  r1 r2 
Equipotentials
• Gravitational equipotentials
are just contour lines: lines
connecting points (x,y) at
the same height.
(Remember PE = mgh.)
• It takes no work against
gravity to move along a
contour line.
• Question: What is the
significance of contour lines
crowding together?
Electric Equipotentials: Point Charge
• The potential from a point charge Q is
1 Q
V r  
4 0 r
• Obviously, equipotentials are surfaces of
constant r: that is, spheres centered at the
charge.
• In fact, this is also true for gravitation—the
map contour lines represent where these
spheres meet the Earth’s surface.
Plotting Equipotentials
• Equipotentials are
surfaces in three
dimensional space—we
can’t draw them very
well. We have to settle
for a two dimensional
slice.
• Check out the
representations here.